The invention relates to a furnace which is for heating up billets, rods, tubes and similar cylindrical charges.
Known from WO No. 83/02661 is a furnace for heating up billets, rods, tubes and similar cylindrical charges which are passed through a treatment chamber by means of a transportation device. This transportation device is, however, of a specific, pre-determined height such that the longitudinal axis of charges of different diameter do not lie exactly in the middle of the treatment chamber. As a result the surface of the charge is non-uniformly jetted by the hot gas stream, which produces non-uniform heating that can cause distortion e.g. curvature of the charges. This, in turn, adversely affects the further processing of the said charge. The said distortion or curvature produces even more pronounced non-uniform heating as the distance between the wall of the cylindrical treatment chamber and the surface of the item being treated varies in an uncontrolled manner.
The ventilator impellers or fans for producing the circulating gas stream are arranged such that the charge being treated is not uniformly heated by the gas stream along its whole length, unless additional, expensive and pressure-reducing constructive means such as deflectors are provided; this non-unformity in heating is due to the impellers being arranged on one side only. Furthermore, the impellers blow the hot gas perpendicular to the longitudinal axis of the charge being treated with the result that recovered pressure in the impeller housing presents problems.
A further disadvantage of this known heating furnace lies in the use of slit-shaped nozzles for the convective heating. These nozzles, arranged along the length of the charge being treated, create narrow jets of hot gas, the exit velocities of which vary in different directions over the periphery of the charge so that non-uniform impingement and hence non-uniform heating around the periphery results.
Corresponding constructions are revealed in the German patent publications DE-OS No. 2'929'322, DE-OS No. 2'712'279, DE-AS No. 2'637'646 and DE-OS No. 2'349'765.
In the light metal industry, for example in the case of cylindrical charges of aluminum, ever increasing demands are being made with respect to the uniformity of heating and the accuracy of holding at the required temperature. Also, for economic reasons it is desireable that the treatment time should be kept as short as possible; consequently, efforts are made to make the thermal gradient as large as possible. However, under conditions of high thermal gradients, local fluctuations in particular have a very pronounced effect, and can lead to local overheating. This is a serious shortcoming in heating furnaces in which the cylindrical charge is heated directly by flames in order to improve the heat transfer conditions, such as is known from the U.S. Pat. No. 3,837,794.
Also known are heating furnaces in which the charge is rotated during treatment (U.S. Pat. No. 4,410,308) or is inductively heated (German patent publication DE-OS No. 2'628'657). The outlay for the necessary constructions, however, is large in such cases.
Furthermore the inductive heating does not guarantee uniform heating of the cylindrical charge over its whole length.
Revealed in the German patent publication DE-OS No. 2'919'207 is a transportation device for a heating furnace, in which, however, the heating of the cylindrical charge takes place by direct contact with heated, refractory blocks i.e. not via convective heating.
A heating furnace of the kind mentioned at the start is revealed in U.S. Pat. No. 4,065,249, and features a treatment chamber the shape of which takes into account the cross-section of the charge, a transportation device for driving the charge in the direction of its longitudinal axis through the treatment chamber, a ventilator or fan to generate a gas stream, a heating device for heating the gas stream, and nozzle outlets to introduce the heated gas stream into the treatment chamber. Here too, however, the above mentioned disadvantage prevails i.e. the non-uniform heating of the cylindrical charge over its length and/or over its periphery, a shortcoming which is due to the non-uniform impingement of the hot gas stream on the item being treated.
The object of the present inventions is therefore to develop a heating furnace of the kind discussed above, in which the above mentioned disadvantages do not occur.
The proposed furnace should in particular produce a defined, uniform heating-up of the cylindrical charge over its whole length and periphery, and eliminate distortion due to non-uniform heating.